The present invention relates to a method for communicating forward error correction (FEC) encoded packet information in a digital telecommunications system, wherein the amount of communication resources is variable for each transmission of information and wherein erroneously received packets may be selectively re-transmitted. Through the inventive method the estimated total time for communicating packets from a transmitting party to a receiving party is minimised.
The invention is also directed to an arrangement for performing the above mentioned method.
The invention is particularly suitable for use in a GPRS system (GPRS=General Packet Radio Service), standardised within GSM (GSM=Global System for Mobile telecommunication).
A packet is defined as an amount of information, which can be sent from a transmitting party to a receiving party over one or more communication resources. A communication resource is typically a channel and may, for instance, be a particular carrier frequency in a FDMA system (FDMA=Frequency Division Multiple Access), a particular time slot in a TDMA system (TDMA =Time Division Multiple Access), a particular code or signature sequence in a CDMA system (CDMA=Code Division Multiple Access), a particular sub-channel carrier in an OFDM system (OFDM=Orthogonal Frequency Division Multiplex) or a certain wavelength in a WDMA system (Wavelength Division Multiple Access). Normally, each packet is segmented into a number of data blocks. A very short packet may be fitted into a single data block, but in most cases a packet corresponds to two or more data blocks.
A packet can further be defined differently on different logical levels in a telecommunications system. Hence, what is one packet on a first logical level may be considered to be several packets on a second logical level. For instance in GPRS higher level packets are usually split into two or more so called LLC frames (LLC=Link Layer Control) before transmission across the air interface. With regard to this invention such LLC frames and corresponding sub-packets are likewise considered as packets.
Moreover, before a data block is sent, redundancy symbols may be included in the data block. The redundancy symbols are correlated with the payload information in the data block, so that a limited deterioration of the data during the transmission may be corrected by the receiving party. The process of adding redundancy symbols is called forward error correction coding and is carried out in accordance with a coding scheme.
The more redundancy symbols that are included, the more transmission deterioration can be tolerated. However, the size of a data block is constant. Thus, a large amount of redundancy results in many data blocks, which of course, gives a longer transmission time than if no redundancy symbols had been added. On the other hand, a small amount of redundancy increases the probability for data block re-transmissions, due to the occurrence of unrecoverable errors during the transmission. A large number of such re-transmissions definitely leads to a long total transmission time for the information contained in the packet.
From the document U.S. Pat. No. 5,526,399 is known as method for transmitting information in a radio communications system, whereby a combination of Forward Error Correction (FEC) and Automatic Repeat reQuest (ARQ) can be used to achieve a good transmission efficiency. The number of repeat communication requests per time interval is monitored and if this number is greater than a first value, then the amount of forward error correction is increased. If instead the number of repeat communication requests per time interval is smaller than a second value the amount of forward error correction is decreased.
A method for non-transparent data transmission is disclosed in WO, A1, 96/36146. The document describes how channel coding is employed on information, which is transmitted from a first to a second party. The quality of the non-transparent connection is monitored and if the quality drops to a certain level, then the channel coding is changed to a more efficient one. In order to compensate for a lower transmission rate per channel for the payload information, during the use of the more efficient channel coding, the allocated channel capacity is simultaneously increased.
The document U.S. Pat. No. 4,939,731 discloses another example of a method, in which an increasing number of repeat communication requests is overcome by expanding the amount of redundancy in each data block in relation to the payload information. Here however, there is no compensation for the lowered payload transmission rate.
In the article xe2x80x9cPerformance of the Burst-Level ARQ Error Protection Scheme in an Indoor Mobile Radio Environmentxe2x80x9d, IEEE Transactions on Vehicular Technology, No. 1, March 1994, pp 1412-1416, E. Malkamxc3xa4ki presents a burst-level ARQ scheme as an alternative to conventional FEC-encoding for speech transmission, where redundancy is sent only when required.
All these methods, in one way or another, adaptively set the relationship between the amount of forward error correction and the number of data block re-transmissions per time interval in order to achieve an efficient transfer of information from a transmitting party to a receiving party.
Nevertheless, none of the previously known methods a priori aims at minimising the total transmission time for the information. Instead, each method tentatively varies the amount of forward error correction to a level, which for the moment gives a satisfying throughput of the payload information.
An object of the present invention is thus to minimise the average transmission time for payload information in a telecommunications system.
Another object of the invention is to utilise the air interface of a radio communications system as efficiently as possible, when communicating packet information in such a system.
A further object of the invention is to provide a reliable transmission of packet information in an environment, where the transmission conditions are unstable.
These objects are met by the present invention by a priori minimising the estimated transmission time for each packet communicated through the system. Generally, the available communication resources have quite varying transmission quality. It is therefore important to make a wise selection from these resources. Depending on how many communication resources that have sufficiently high transmission quality and the quality variance between these resources, one particular coding scheme is the optimal scheme to use. Thus, it is crucial which coding scheme that is chosen in combination with the allocated communication resources. Moreover, the distribution of encoded data blocks over the allocated communication resources is a parameter, which must be optimised.
According to one embodiment of the present invention there is provided a method for communicating packet information in a digital telecommunications system. The method presupposes that the amount of communication resources is variable for each packet and that the system operates in accordance with a protocol, which admits selective re-transmission of erroneously received packets. Furthermore, it must be possible to forward error correction encode the payload information via one of at least two different coding schemes, before sending the information to a receiving party.
The method finds, for each packet and set of available communication resources, a combination of a coding scheme and a subset of resources, which minimises an estimated transmission time for the information contained in the packet. By transmission time is meant, the time from start of transmission of an initial data block in the packet, to reception of a positive acknowledgement message for the packet, i.e. including any possible intermediate delays and waiting times. Naturally, the choice of coding scheme and subset of resources may also be based upon additional premises, on condition that the above mentioned transmission time-requirement is met. After having found such an optimal combination of a coding scheme and a set of communication resources the packet is encoded via this coding scheme and transmitted to the receiving party over these resources.
According to another embodiment of the present invention there is provided a method for communicating packet information in a digital telecommunications system as defined above, through which, for each packet and set of available communication resources, there is found a combination of a coding scheme and a distribution of encoded data blocks, over a subset of the available resources.
The method comprises the following non-consecutive steps. Determining which, of all communication resources in the system, that are presently available for transmitting a particular packet; selecting from the available communication resources a designated set, which contains the most suitable resources for sending the packet; estimating a transmission time, for sending two or more encoded versions of the packet, over the designated set of resources; and selecting a combination of a coding scheme and a distribution of encoded data blocks, that minimises the estimated transmission time, for sending the encoded data blocks, which constitute an encoded version of the packet. The distribution thus defines exactly which encoded data block that shall be transmitted over which communication resource. Finally, the packet is encoded via the coding scheme of the selected combination and transmitted over the communication resources of the designated set, according to the distribution, which in combination with the selected coding scheme, is expected to be optimal from a transmitting time-point of view.
According to yet another embodiment of the present invention there is provided a method for communicating packet information in a digital telecommunications system as defined above, through which, for each packet and set of available communication resources, there is found a combination of a coding scheme and a distribution of encoded data blocks, over a subset of the available resources.
The method comprises the following consecutive steps. First, determining which, of all communication resources in the system, that are presently available for transmitting a particular packet. Second, deriving an estimated transmission quality for each of these communication resources. Third, selecting from the available resources a designated set, containing the most suitable resources for sending the packet. Fourth, selecting an arbitrary coding scheme from the coding schemes, that are available within the system. Fifth, calculating a number of encoded data blocks that would be comprised in an encoded packet, which is derived from encoding the packet via the selected coding scheme. Sixth, selecting a first relevant distribution of encoded data blocks, which would be obtained from the encoding process, over the communication resources of the designated set. Seventh, calculating an estimated transmission time for such a transmission. Eighth, repeating the distribution, encoding and calculating steps until an estimated transmission time has been established for each combination of coding scheme and relevant distribution of encoded data blocks.
By relevant distribution is meant that a communication resource having a high estimated transmission quality always shall transmit at least as many encoded data blocks of a particular encoded packet, as a communication resource having a lower estimated transmission quality. However, a relevant distribution may very well mean that no encoded data blocks at all are distributed on one or some of the resources, which are identified in the designated set. A relevant distribution may also imply that the designated set contains no more resources than what can be handled by the least competent party involved, transmitter or receiver respectively.
Ninth, selecting an encoded packet which, in combination with a certain distribution, is expected to minimise the transmission time for the payload information contained in the original packet. Tenth, transmitting this encoded packet over the designated set of resources, according to this distribution.
An arrangement according to the invention for communicating packet information in a digital telecommunications system comprises the following: a buffer means, for storing data blocks, that constitute a particular packet; an encoding means, for retrieving a packet from the buffer means and producing therefrom an encoded packet; a computing means, for (i) deriving quality measures for all currently available communication resources, (ii) determining a designated set of communication resources from those resources that are available, (iii) determining relevant distributions of encoded data blocks over the designated set of communication resources and (iv) calculating estimated transmission times for sending an encoded packet according to at least two different combinations of encoding and distribution; and a transmitting means, for sending an encoded packet to a receiving party, over at least one of the communication resources of the designated set of resources, according to a distribution, which in combination with a particular encoding, is expected to give the shortest transmission time for the encoded packet.
According to an aspect of the present invention there is provided a base station control unit for a radio telecommunications system, e.g. GPRS, which comprises the inventive arrangement.
According to another aspect of the present invention there is provided a switching unit in a packet switched telecommunications network comprising the inventive arrangement.
The arrangement according to the invention may, of course, also be comprised any other resource allocating unit in a mobile or stationary digital telecommunication system.
The present invention thus offers a solution to the problem of minimising the average transmission time for payload information in a telecommunications system by, for each packet, selecting a combination of a forward error correction coding scheme and a block distribution over the available communication resources, that minimises the estimated transmission time for the packet.
Given an accurate estimate, the transmission time for any piece of information will so, on average, be minimal.
Consequently, a radio communications system, which applies this method for packet communication across its air interface, will also utilise the air interface as efficiently as possible, since the method minimises the total amount of radio resources necessary per piece of information being communicated.
The proposed solution also ensures a reliable transmission of information in an environment, where the transmission conditions are unstable. Since, regardless of any possible variations in the transmission qualities of the individual communication resources, the currently most efficient combination of forward error correction coding scheme and block distribution over the available communication resources is always selected.